>>839-841
SVP Managerを起動していないと赤枠が出るというのはSVPflow 4.3からの仕様みたいだね。

https://www.svp-team.com/wiki/Plugins:_SVPflow
> Please note that SVPflow libs require a SVP Manager running (Windows and macOS only),
> otherwise you'll see a red rectangle around the video frame.

https://www.svp-team.com/wiki/Download#libs
> SVPflow 4.3.0.168
> Windows, macOS: running SVP Manager is required!

svpflow-4.3.0.168のreadme.txt
> WARNING: SVPflow libs require a SVP Manager running, otherwise you'll see a red rectangle around the video frame.
> You're free to disable SVP completely ("Temporarily disable SVP" + ui.disable_on_launch = true in All settings),
> however it still must be running in background.
　
　
少し前の4.2.0.142ではこれらの記述は無かったので、4.2.0.142を使えば赤枠は出ないと思う。バイナリはWebArchiveから落とせる。

　　https://web.archive.org/web/20190526051617/http://www.svp-team.com/files/gpl/svpflow-4.2.0.142.zip